This invention relates to a glass-ceramic substrate for a magnetic information storage medium used in an information storage device and, more particularly, to a glass-ceramic substrate for a magnetic information storage medium such as a magnetic disk substrate having a highly smooth substrate surface suitable for use in a near contact recording or a contact recording used mainly in the ramp loading system and also having a high Young's modulus and low specific gravity capable of coping with a high speed rotation of the storage medium. The invention relates also to a method for manufacturing this glass-ceramic substrate for a magnetic information storage medium. This invention relates further to a magnetic information storage medium made by forming a film on such glass-ceramic substrate for a magnetic information storage medium. In this specification, the term "magnetic information storage medium" includes fixed type hard disks, removable type hard disks and card type hard disks used respectively such as so-called hard disks for personal computers and other magnetic information storage medium in the form of a disk which can be used for storage of data and can be also used in digital video cameras and digital cameras.
In contrast to conventional fixed type magnetic information storage devices, there have recently been proposed and put into practice magnetic information storage device of a removable type or a card type and developments of applications thereof for digital video cameras and digital cameras have been started. Such tendency relates to utilization of personal computers for multi-media purposes and prevailing of digital video cameras and digital cameras and, for handling data of a large size such as data of a moving image or voice, there is an increasing demand for a magnetic information storage device of a larger storage capacity. For coping with this demand, a magnetic information storage medium is required to increase its bit and track density and increase the surface recording density by reducing the size of the bit cell. On the other hand, as for a magnetic head, there is an increasing tendency toward adopting the near contact recording system and further the contact recording system according to which the magnetic head operates in closer proximity to the disk surface in accordance with the reduction of the bit cell size.
Aluminum alloy has been conventionally used as a material of a magnetic information storage medium substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness and smoothness as the above described magnetic information storage medium substrate of high recording density. Further, since the aluminum alloy is a soft material, its Young's modulus and surface hardness are low with the result that a significant degree of vibration takes place during rotation of the disk drive at a high speed rotation which leads to deformation of the disk and hence it is difficult to cope with the requirement for making the disk thinner. Furthermore, the disk tends to be deformed due to contact with the magnetic head with resulting damage to the contents of the disk. Thus, the aluminum alloy cannot cope sufficiently with the requirement for recording with a high recording density.
As a material for overcoming the above problems of the aluminum alloy substrate, known in the art are chemically tempered glasses such as alumino-silicate glasses (SiO.sub.2 --Al.sub.2 O.sub.3 --Na.sub.2 O) disclosed in Japanese Patent Application Laid-open No. Hei 8-48537 and Japanese Patent Application Laid-open No. Hei 5-32431. This material, however, has the following disadvantages:
(1) Since polishing is made after the chemical tempering process, the chemically tempered layer is seriously instable in making the disk thinner. Further, the chemically tempered layer causes change with time after use for a long period of time with resulting deterioration in the magnetic property of the disk. PA0 (2) Since the chemically tempered glass contains Na.sub.2 O and K.sub.2 O as its essential ingredients, the film forming property of the glass is adversely affected, and a barrier coating over the entire surface of the glass becomes necessary for preventing elution of Na.sub.2 O and K.sub.2 O ingredients and this prevents stable production of the product at a competitive cost. PA0 (3) The chemical tempering is made for improving mechanical strength of the glass but this basically utilizes strengthening stress within the surface layer and the inside layer of the glass and hence its Young's modulus is about 83 GPa or below which is about equivalent to ordinary amorphous glass. Therefore, use of the glass for a disk with a high speed rotation is limited and hence it is not sufficient for a magnetic information storage medium substrate.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some glass-ceramic substrates. For example, Japanese Patent Application Laid-open No. Hei 9-35234 and EP0781731A1 disclose glass-ceramic substrates for a magnetic disk made of a Li.sub.2 O--SiO.sub.2 system composition and has crystal phases of lithium disilicate and .beta.-spodumene, or crystal phases of lithium disilicate and .beta.-cristobalite. In these publications, however, the relation between Young's modulus and specific gravity with respect to high speed rotation is not taken into consideration or suggested at all. The upper limit of Young's modulus of these glass-ceramics is no more than 100 GPa.
For improving the Young's modulus, Japanese Patent Application Laid-Open No. Hei 9-77531 discloses a glass-ceramic of a SiO.sub.2 --Al.sub.2 O.sub.3 --MgO--ZnO--TiO.sub.2 system which contains a large amount of Spinel crystal as its predominant crystal phase and MgTi.sub.2 O.sub.5 and several other crystals as its subordinate crystal phases and has Young's modulus of 93.4 GPa to 160.11 GPa and a substrate for a high-rigidity magnetic disk made of this glass-ceramic. The glass-ceramic has, as its predominant crystal phase, Spinel crystal represented by (Mg/Zn) Al.sub.2 O.sub.3 and/or (Mn/Zn).sub.2 TiO.sub.4 containing a large amount of Al.sub.2 O.sub.3 and has several other optional crystals as its subordinate crystal phaes. This glass-ceramic is significantly different from the glass-ceramic of the present invention which, as will be described later, contains a relatively small amount of Al.sub.2 O.sub.3 and has a high Young's modulus and a low specific gravity. Addition of such a large amount of Al.sub.2 O.sub.3 is undesirable from the standpoint of production because it causes deterioration of melting property of the base glass and also deterioration of resistance to devitrification. Further, in this publication, the relation of Young's modulus (GPa)/specific gravity and the value of specific gravity per se which are necessary for the high speed rotation are not taken into consideration or suggested at all. Particularly, the specific gravity in this publication is a high value of 2.87 or over. This publication therefore merely proposes a glass-ceramic substrate made of a rigid material. Moreover, the glass-ceamic of this system has the serious disadvantage that processability is poor and therefore is not suited for a large scale production since it is too hard and hence the improvement of this glass-ceramic as a high recording density magnetic information storage medium substrate is still not sufficient.
It is therefore an object of the invention to eliminate the above described disadvantages of the prior art materials and provide a glass-ceramic substrate for a magnetic information storage medium capable of coping with the tendency toward a high recording density recording, namely having excellent melting property, resistance to devitrification and processability, excellent smoothness of the surface of the substrate which is sufficient for contact recording of a high recording density and having a high value of Young's modulus and a low value of specific gravity which are sufficient for a high speed rotation of the disk.
It is another object of the invention to provide a magnetic information storage medium made by forming a film of a magnetic medium on this glass-ceramic substrate.